PRACTICAL EXERCISES 23 



Treat another preparation, in which active movement is going 

 on, with iodine solution : the movement will be arrested, the 

 cell being killed : the protoplasm will be stained brown. 



Similar movements of rotation are to be seen more or less 

 clearly in living cells generally, and are easily observed in 

 cells of the leaf of Vallisneria spiralis, Elodea canadensis, and 

 especially well in the large internodal cells of Nitella, &c. 



More complicated movements are to be seen in various hairs, 

 and notably in those which cover the base of the stamens in 

 species of Tradescantia. Remove a few of the hairs from a 

 stamen of an open flower, and mount them in water. Observe 

 under a low power the moniliform hairs, each composed of a 

 row of barrel-shaped cells. Focus the high power upon one of 

 these cells, and note the limiting cell-wall, and protoplasmic 

 lining : threads or bridles of protoplasm, irregularly disposed, 

 pass from the peripheral protoplasm towards the centrally dis- 

 posed, spherical nucleus. 



Examination of these threads will disclose movements of 

 the protoplasm in various directions : these more complicated 

 movements are collectively termed circulation. The hairs 

 should be treated as above directed in the case of the root- 

 hairs, to show that the movement depends upon the life of 

 the cell. 



